Following increased demand for distribution of high-resolution moving picture and the popularization of bidirectional real time image services represented by a video phone in recent years, speedup and capacity increase beyond 100 Tbps is desired for a future optical communication network. In order to realize speedup and capacity increase of an optical communication network, it is necessary to use a frequency more effectively in addition to speedup in a time axis and widening in a frequency axis. For example, non-patent document 1 has proposed a communication system aiming at improvement of utilization efficiency of a frequency. In this optical communication system, the frequency utilization efficiency is improved by optimization of the multiplex mode of the optical communication system, and a multiplexing/demultiplexing apparatus having a variable pass frequency band is used to enable maximum use of the above-mentioned multiplex mode. As a result, in an optical communication system disclosed in non-patent document 1, further high-efficient use of a frequency can be realized.
Patent document 1 proposes, about a multiplex method for high-efficient use of a frequency used by an optical communication system, a method which combines both of an orthogonal frequency division multiplex method used for wireless transmission and a coherent communication method. Hereinafter, the orthogonal frequency division multiplex method is described as OFDM. OFDM is one of frequency (that is, wavelength) division multiplex methods the same as a WDM (wavelength division multiplexing) communication method that is the mainstream in optical communication systems. By orthogonalizing a plurality of channels for sending data with each other, OFDM permits overlap on a frequency. For this reason, by adopting OFDM, the utilization efficiency of a frequency in an optical communication system can be improved.
Also, in the OFDM technology, in order to separate a plurality of orthogonalized channels a coherent communication technology is used. A digital coherent receiver used in the coherent communication technology takes out intensity information and phase information on light using local light, and performs channel separation by digital signal processing using a DSP (digital signal processor). Here, at the time of channel separation in OFDM, it is desired that loss in each channel is constant with respect to frequencies. Accordingly, for optical parts arranged in a midway between a transmission unit and a reception unit of an optical communication system adopting OFDM, it is required that transmission characteristics in which loss in respective channels used by OFDM is constant and wavelength characteristics of loss in a channel is flat.
Described below are technologies for realizing flat transmission characteristics in which a bandwidth to be used can be selected freely, and loss in a plurality of channels used by OFDM is constant.
Non-patent document 2 discloses a phase controlling device using LCOS (liquid crystal on silicon). A phase controlling device disclosed in non-patent document 2 includes a collimate lens, a grating and a spatial optical system using LCOS. The grating separates signals of different frequencies spatially, and the LCOS operates as an optical switch by performing phase control of the signals by a liquid crystal. The phase controlling device disclosed in a non-patent document 2 realizes band-variable characteristics and wavelength-variable characteristics by controlling liquid crystal elements used for an optical switch.
Patent document 2 discloses a structure of an optical variable filter equipped with a function that changes a wavelength and a bandwidth independently. The optical variable filter disclosed in patent document 2 has a device of a spatial optical system consists of a grating and a reflector. By performing fine position control of reflectors mechanically, respectively, using a mechanical structure such as a stepping motor, the optical variable filter disclosed in patent document 2 performs variable control of a wavelength and a band.
Compared with a phase controlling device using LCOS, the optical variable filter disclosed in patent document 2 has a merit that a variable range of a wavelength and a band can be wide and it can be manufactured inexpensively. By combining such optical variable filter with a 1×n coupler which is an optical branching part, a 1×n multiplexer/demultiplexer having variable band characteristics and wavelength characteristics can be realized.
Patent document 3 describes a structure of a wavelength multiplexing separation filter which is made by combining an AWG (arrayed waveguide grating) and an interleaver. The wavelength multiplexing separation filter disclosed in patent document 3 realizes a wavelength multiplexing and separating filter by employing an AWG as a wavelength multiplexer/demultiplexer, and overlapping a plurality of pieces of light to which wavelength separation has been performed.
In addition, patent document 4 describes a structure of a high-frequency generator made by combining a frequency branching filter and an optical switch. Patent document 5 describes a structure with which spectrally separated light rays are multiplexed by a star coupler in a light wavelength multiplexing apparatus.